I am hoping this isn't too wide of a topic since garage storage can vary greatly from person to person depending on what's being stored. In my case, I will be storing power tools like my circular saw, reciprocating saw, heavy toolbox full of misc hand tools, boxes of screws, paint cans, cases of water, 50-lb bags of kitty litter, etc.

Update 12/21/2012

After much consideration, I decided against the wall-mounted 16x18 shelf brackets that I had already hung per Amanda's suggestion and went with free-standing shelves as recommended by alx9r. He provided a logical argument against supporting large amounts of weight on a residential wall frame and it honestly made sense. Plus, the cross bar on those brackets just takes away from your available storage space. I will say, though, that these Everbilt brackets were super sturdy when fastened with heavy duty #10 screws. I could jump up and down on those shelves and they'd stay put. Here's a pic of the shelves I installed using the brackets:

And here are the adjustable shelves by Sandusky that I bought from Sam's Club:

The only think I don't like about these is that the shelves need more support. Being that they're 6' wide, they bow pretty badly if you set a heavy load right in the center. However, I am sure that there's a way to reinforce them, but I will worry about that later down the road. For now, each shelf supports 600 lbs, more than adequate for my needs. The vertical rods also support caster wheels, although I will need heavy duty threaded stem casters not available at any of the big box stores around here for a reasonable price.

Anyway, I hope this update helped.

Edit 12/3/2012

I bought a bunch of the brackets suggested in Amanda's answer and have since installed one shelf. It is very sturdy. The downside to these brackets, though, is how the diagonal support bar takes up a large amount of space. If you've mounted the brackets every 16", this keeps you from storing large objects wider/longer than 16" on the shelf below. The solution to that is to skip every other stud and mount the brackets every 32", but then you have the issue of sagging. Originally, I bought 18"x96"x0.75" lengths of MDF, but this will not work for 36" support spacing, so I will need to get stronger wood like plywood. However, there will still be some sagging involved at 32", so it appears that BrianK's suggestion to put a 1x2 along the edge will give it more support. Is there a technical term for such a strip? The term "shelf edging strip" seems to come up in Google searches, but I don't see a lot of products designed for this. I'd like to see some sort of c-shaped metal strip on the market that fits snug along the edge of a board rather than having to glue/screw a 1x2 onto the edge of the shelf.

The other option is to use use vertical shelf tracks and then mount adjustable shelf brackets onto them. This would allow 16" bracket spacing with no cumbersome support bar to get in the way. The problem is that I cannot find any heavy duty, adjustable brackets that work with plywood that support in the neighborhood of 300 lbs per bracket.

So, in summary, what I like about Amanda's brackets are their 650-lb weight capacity and ability to support a 16"-deep shelf. The downside is that support bar. The adjustable shelves, on the other hand, don't have such a support bar. Unfortunately, they can only hold around 100 lbs per bracket. Is there a way to get the best of both worlds here or am I going to have to build shelving from scratch? I definitely do not want to do that.

The fasteners may be the limiting factor. The top fasteners will have significant withdrawal load. What is the stud spacing? 48" bracket spacing is probably stretching things, 32" is probably OK, depending on MDF thickness and number of fasteners. If you want real heavy duty, you shouldn't hang the shelves at all.
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bcworkzNov 24 '12 at 23:11

3/4" MDF. I will check out bigger brackets tomorrow. What do you mean by "you shouldn't hang the shelves at all"? You mean I should look into perhaps tiered shelving units? Again, I am sure it comes down to "what is heavy duty in my scenario". I don't need to support thousands of pounds. Just hundreds.
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oscilatingcretinNov 24 '12 at 23:18

3/4 MDF with brackets spaced at 32" will not hold your 50lbs kitty litter without sag. 32" spacing for a MDF shelf is scary.
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GunnerNov 25 '12 at 0:20

1

@oscillatingcretin I've had luck with specialty heavy duty casters at mcmaster.com. I just retired three of the same shelves as your pictures with swivel casters. You might want to consider that casters increase the risk of toppling significantly for two reasons (1) the offset design common to most casters can move the contact point with the floor closer to the center of gravity, and (2) rolling means that you now have a live load whose center of gravity is much higher than a braking force that would typically be applied at the bottom of the shelf (think caster running into a stick).
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alx9rJan 8 '13 at 0:50

What is the Maximum Weight I will be Storing?

With all 24 on the shelves, I physically cannot add any more weight to the shelves because all of the volume is already consumed.

I estimate the maximum weight that I put in each of those bins is about 50 pounds of stuff. From moving them around and comparing them to the dumbbells at the gym, it's way more that 20 pounds and definitely less than 100 pounds. As a reference, filling the tub with water would weigh 81 lbs, filling with lead is about 946 pounds.

So, based on my estimate, I will be storing up to about 1200 pounds (24 tubs x 50 pounds each) of stuff on that shelving unit. More than 1200 pounds is unlikely because the shelf will reach its volume capacity first.

In this case I dare not use a shelving unit that will collapse under less weight than 1200 pounds because there will be nothing preventing an unsuspecting family member from exceeding that weight by adding another item to the shelf.

The shelving unit in the example above is 4'x2'x8' (WxDxH) or 64 cubic feet. Given the 1200 pound capacity, the density of stuff I am storing works out to 18.75 pounds per cubic foot. When I design shelving for my home, I use the round number of 40 pounds per cubic foot for a 2x safety factor.

For your example of 1.5'x8' shelves, assuming they extend to an 8' ceiling, you should design for an astounding 3840 pounds of load (1.5'x8'x8'x40 pounds per cubic foot) to be safe. This is not an exaggeration. Good shelving allows you to store a remarkable amount of weight in an extremely small volume, that is its whole purpose after all.

Why Wall-Mounted Shelving is the Wrong Approach for High Density Heavy Storage

The stuff you are storing on wall mounted shelving is an overhung load. The wall mount shelving supports this overhung load by applying a torque to the wall to which it is mounted. Walls in residential construction are not designed to support a significant amount of overhung load.

To make matters worse, the torque on the wall increases with the square of the depth of the shelving because you are increasing both the weight and the lever arm of the overhung load. So, the torque from an 18" deep shelf is nine times the torque from a 6" deep shelf. Because of the squared relationship with torque, I never use wall-mount shelves deeper than 10".

On top of all this, even if your wall is able to withstand the torques involved, there are two additional worrisome aspects to consider:

The wall is now supporting an additional several hundred or thousands of pounds of weight vertically -- something that it was surely never designed for.

There are extremely high multiplied forces where the horizontal shelf brackets meet the wall as a consequence of leverage -- anywhere you find high forces like this, you should also expect catastrophic failure.

A real risk is this scenario: A child climbs a wall mount shelf loaded with hundreds of pounds of stuff. The weight and motion of a child causes the multiplied forces in the bracket to exceed the failure strength of the bracket. This causes a catastrophic failure of the shelf's ability to support the load, and the child and load falls to the floor, crushing the child.

Use Free-Standing Modular Shelving Instead

All of the problems that results from multiplying forces, the squared relationship of torque with shelf depth, and a wall that can't support weight are alleviated by using free-standing shelving.

I recommend using a properly engineered system like EZ Rect Type 1. It looks like this:

As I understand it, the EZ Rect system's patent has long expired, and there are now many manufacturers of that system. I've been using a generic version of that system for all my high-density household storage for almost 10 years.

Here are the benefits of EZ Rect Type 1 shelving solution as I see it:

each "bay" is usually specified to support in the range of 7000 lbs, so there is very little risk of overloading your shelf with household items (remember you'll hit the volume limit first)

the only real risk of catastrophic collapse is by racking or toppling, both of which can be mitigated by fastening the shelf to a wall

there are multiple manufacturers and distributors of the same compatible system which means you can buy compatible pieces as you go (you may want to confirm compatibility before investing heavily)

you can add and remove shelves and adjust shelf spacing after the shelf has been erected

there are no diagonal supports that interfere with access or limit storage

the "shelf" part of the system can be bought and cut from the most cost-effective sheathing available at your Home Depot/Lowes/etc

the dimensions of the "shelf" part are a round number of inches, which makes it easy to get someone to cut the shelving using a panel saws at the store

the dimensions of the "shelf" usually can be evenly cut from a 4'x8' sheet

the "post connectors" and "frame connectors" can be mix and matched to achieve your desired combination of width and depth

the "posts" come in different heights and can be custom cut to length with a hack saw

if you don't need the shelf anymore, it collapses to a very compact size

erection and dismantling requires only a hammer and is quite quick

you take the shelving with you when you move, and it is easy to move when dismantled

Whatever system you end up with, you should expect the above benefits to be standard -- if it's not, keep looking.

Excellent answer! Very clear explanation of the issues of hanging or not hanging shelves. These kit shelves are competitive with raw materials, available in different weights and are very sturdy. Heartily recommended.
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Tim QuinnDec 9 '12 at 7:11

Good answer indeed, but I am skeptical about the emphasis on overhung loads in residential construction being that bad of an idea. If the frame really wasn't suitable for bearing heavy loads, I strongly doubt companies would be manufacturing RSI clips designed to suspend up to 4 layers of 5/8" drywall directly on studs in residential soundproofing applications. Also, I won't be storing thousands of lbs per bracket. At most, I'd say 600-700 lbs will be dispersed across all three 16"x96" shelves. Each bracket is also mounted with three 2 1/2" heavy duty #10 screws. It's very sturdy.
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oscilatingcretinDec 9 '12 at 20:00

@osc I don't follow your logic wrt the existence of certain products meaning something about load capacities. In general, the load capacities of wall assemblies depend on a variety of factors - that is why there are tables of tested assemblies, architects, and structural engineers. Some structures are designed to support heavier loads by increasing stud size, density, etc. Most residential structures are designed to support the original construction only. Furthermore, the moment arm of the overhung load of your drywall scenario is an order of magnitude less than shelving would be.
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alx9rDec 9 '12 at 22:38

@osc wrt the amount of weight you are storing. A safe shelf design would contemplate the maximum weight a haphazard combination of users are likely to store. Users do not think about how much weight a shelf can support when they put something on it - only whether it fits. That is why being volume-limited is preferable. Storage system collapse is common in industry. The scenario is usually that a shelf system gets overloaded and collapses because of a change of use a long time after it is built.
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alx9rDec 9 '12 at 22:47

I know for a fact that I will never come close to reaching weight capacity. In fact, I've almost reached volume capacity with about 300-400 pounds' worth of stuff on all three shelves combined. A homeowner would have to be pretty ambitious to exceed capacity of these shelves to the point where the full two inches of the upper two #10 screws of each of the four brackets are all yanked clean out of the wall at the exact same time without even so much as the slightest warning that structural integrity has been compromised. In short, they're not going anywhere.
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oscilatingcretinDec 9 '12 at 23:53

Don't use brackets. use two by threes to make shelves of the size you want. frame with them edge up and screw 1/2' or 3/4" plywood on top. bolt the back edge into every stud. every five or six feet on the edge furthest from the wall bolt in another 2 by three that is oriented vertically and goes all the way to the floor. I support a very large amount of weight on shelves with this system. if the span ends up too big and sagging occurs you can stiffen the shelf by adding another two by three behind the outermost one.

Everbilt makes deeper brackets with a cross piece for stability. I got ours at Ace, but Home Depot carries them, too.

Whether or not the MDF is strong enough really depends on the MDF. It is surely strong enough if you hit every stud, so you could mount braces at every third stud and start filling the shelves. If they sag even a little, add more braces.